Smoke and fog generators are used in a variety of applications such as in security applications, for simulating fire as a training aid, or in entertainment, e.g. for creating special effects or lighting effects on stage. In the entertainment the smoke or haze created by smoke generators are especially important for the smoke effect but also essential in creating lighting and lighting effects, where some lightings effects like mid-air light beam effects only become visible when used together with a fine diffused haze.
A smoke generator in general works by a smoke liquid being driven into a heat exchanger to vaporize. When ejected into the ambient, the vaporized smoke liquid condensate and forms a smoke. Different types of smoke or fog generators exist capable of creating everything from a thick heavy smoke or fog to the finest barely visible haze.
Here and in the following the term ‘smoke’ and ‘smoke generators’ will be used in general to describe both smoke, fog and haze, and a generator for creating a smoke, fog, or haze effect. Likewise, the term ‘smoke liquid’ is here and in the following used as a general term for a liquid, which after vaporization and subsequent condensation to micro droplets forms a smoke.
The quality or characteristics of the generated smoke depends in particular on the type of smoke liquid used, which then yields different requirements to the temperatures needed to vaporize the liquid, to the applicable heat exchanger design, and to the means for ensuring a sufficient smoke liquid flow into the heat exchanger.
Different types of smoke generators exist with different means for driving the smoke liquid into the heat exchanger, such as a simple air pump, or mixing the smoke liquid with a compressed or pressurized gas, or pressurizing the smoke liquid itself. It is important to drive the smoke liquid continuously into the heat exchanger under sufficient high pressure to ensure a complete vaporization and to create the liquid flow necessary to create the desired amount of smoke.
It is known to use a constant externally applied pressure from a gas cylinder, which upon connection to the system is throttled down through e.g. a metering orifice to provide the desired flow to the heat exchanger. In most applications, the gas flow is manually controlled to allow some variation in output. However, such generators often encounter problems of variations in output, as the pressure in the gas cylinder reduces.
Also, the flow through the heat exchanger can be seen to be unsteady due to the backpressure created by the vaporisation process of the smoke liquid. This leads to variability in the flow through the unit which is then perceived as variability in the output making the resulting haze uneven, and reducing the quality of the effect when seen with normal stage lighting. The problem of uneven smoke generation is seen to increase with increased smoke liquid delivery, i.e. with more dense smoke generation. The problem may to some extent be alleviated by using a gas supply of sufficiently high pressure at all times which however is economically and environmentally undesirable. Such a system is disclosed in WO 2009/151375 where a supply of nitrogen containing gas is fed at a constant high pressure of at least 6 bar (87 psi) to the smoke generating section as set by a manually or electrically controlled pressure regulator.
Existing smoke generators where the gas is normally delivered at a relatively high pressure are further seen to have problems of blockages in the heat exchangers, which may occur if the desired smoke output is reduced to very low levels. To avoid such blockages, the controls of the machines are often programmed not to allow smoke outputs below a certain level reducing the performance possibilities of the machine correspondingly.